1. Field of the invention
The present invention generally relates to duplexers, and more particularly, to a duplexer that employs an acoustic wave device such as a surface acoustic wave device or a piezoelectric thin-film device.
2. Description of the Related Art
In recent years, as the mobile communication systems have developed, mobile phone handsets and mobile information terminals have rapidly spread, and the manufacturers are still competing against each other for smaller terminals with higher performance. Also, there are both analog and digital systems for mobile telephone communication, and the frequencies used in the motile communication systems range from the 800 MHz to 1 GHz band to the 1.5 GHz to 2.0 GHz. Antenna duplexer devices that employ surface acoustic wave filters have been suggested as the devices for the mobile communication.
In the recent development of mobile phone handsets, the functions of the terminals have been improved with the “dual mode” (the combination of analog and digital systems, and the combination of digital TDMA (time division multiple access) and CDMA (code division multiple access)) or the “dual band” (the combination of the 800 MHz band and the 1.9 GHz band, or the combination of the 900 MHz and the 1.8 GHz or the 1.5 GHz), as the variety of systems has become wider. The components (filters) for those mobile communication systems have also been improved for higher performance.
Meanwhile, other than the functions with higher performance, there is a demand for smaller devise and lower production costs. Each of the antenna duplexers in high-performance terminals is often formed with dielectrics or complex duplexers of surface acoustic wave filters having at least one dielectric, or only with a surface acoustic wave device.
With dielectric duplexers, it is difficult to obtain a smaller or thinner mobile terminal, because of the large size of each dielectric duplexer. Even in a case where a surface acoustic wave device is employed, the size of a dielectric device makes it difficult to obtain a smaller or thinner mobile terminal. Among conventional duplexer devices that employ surface acoustic wave filters, there are module-type devices each having a transmission filter, a reception filter, and a matching circuit mounted separately on a printed board, and integrated-type devices each having transmission and reception filter chips mounted in a multi-layer ceramic package and a matching circuit provided in a package. The volume of each of those devices ⅓ to 1/15 of the volume of a dielectric duplexer, and the height is ½ to ⅓ of the height of a dielectric duplexer. Thus, smaller and thinner duplexer devices can be obtained.
Next, a conventional duplexer is described. FIG. 1 is a block diagram illustrating the structure of the conventional duplexer. FIG. 2 shows the frequency characteristics of the duplexer. In the graph of FIG. 2, the abscissa axis indicates the frequency (which becomes higher toward the right), and the ordinate axis indicates the pass intensity (which becomes higher toward the top). As shown in FIG. 1, the duplexer 10 includes two filters 12 and 13, a phase matching circuit 11, a common terminal 14, and individual terminals 15 and 16. The common terminal 14 connects an external terminal for transmitting and receiving electric waves through an antenna. The transmission terminal 15 is connected to an external transmission circuit, and signals with desired center frequencies are input through the transmission terminal 15. The reception terminal is connected to an external reception circuit, and signals with desired center frequencies are output through the reception terminal 16. The transmission terminal 15, the reception terminal 16, and another terminal are grounded to a ground level (GND).
Normally, the surface acoustic wave filters 12 and 13 and the phase matching circuit 11 are housed in a multi-layer ceramic package. The filters 12 and 13 are surface acoustic wave filters, and have passband center frequencies F1 and F2 that are different from each other (F2>F1). For example, the filter 12 is a filter for transmission, and the filter 13 is a filter for reception (hereinafter, the filters 12 and 13 will also be referred to as the transmission filter and the reception filter). In a duplexer of the 1.9 GHz band, for example, the frequency difference between F1 and F2 is approximately 100 MHz.
The phase matching circuit 11 is provided so as to prevent the filters 12 and 13 from degrading the filter characteristics of each other. Here, the characteristic impedance in the case where the filter 12 is seen from the common terminal is denoted by Z1, and the characteristic impedance in the case where the filter 13 is seen from the common terminal 14 is denoted by Z2. By virtue of the phase matching circuit 11, in a case where the frequency of a signal to be input through the common terminal 14 is F1, the characteristic impedance Z1 on the side of the filter 12 is the same as the characteristic impedance value of the common terminal 14, and the characteristic impedance on the side of the filter 13 is infinite, with the coefficient of reflection being 1. In a case where the frequency of a signal is F2, the characteristic impedance on the side of the filter 12 is infinite, with the coefficient of reflection being 1, and the characteristic impedance Z2 of the filter 13 is the same as the characteristic impedance of the common terminal 14.
Next, the prior arts suggested for smaller devices are described. Japanese Unexamined Patent Publication No. 2004-328676 discloses a duplexer in which the chips of surface acoustic wave filters are face-down mounted to the die-attach layer of the package, and the line pattern for phase matching are pulled over two layers. Japanese Patent No. 3487692 (Japanese Unexamined Patent Publication No. 9-98046) discloses a surface acoustic wave duplexer that has lumped-constant inductors and capacitors in a phase matching circuit.
In Japanese Unexamined Patent Publication No. 2004-328676, however, the package size is as large as 5 mm in side, and 1.5 mm in height, because the line patterns for phase matching are pulled over two layers. Accordingly, the duplexer disclosed in Japanese Unexamined Patent Publication No. 2004-328676 has the drawback of being tall. Also, since a large-area ground pattern (a mat ground) is disposed immediately below the signal lines, the stray capacitance of the signal lines increases, resulting in poorer impedance matching. As the impedance matching deteriorates, the insertion loss increases.
Also, in Japanese Patent No. 3487692, the duplexer is designed to be smaller than a device using dielectric filters. Therefore, the estimated device size is 8×5.1×2.5 mm. Furthermore, Japanese Patent No. 3487692 does not concern the technique of mounting chips in a package, especially the technique of obtaining higher isolation between signals and adjusting the inductance of the ground for parallel-arm resonators.